01 July, 2020
Light reactions: make ATP + NADPH for Calvin cycle


Calvin cycle


Calvin cycle is a 3-step process

- 1. Fixation: CO2 reacts with 5C molecule (RuBP)
- catalyzed by Rubisco enzyme
- new 6C split into 2 acids (3C each)
- 2. ATP and NADPH turn 3C acids into sugars
- 3. Some 3C sugars recycled to make RuBP (5C)
- one 3C sugar leaves to make glucose
- recycling needs ATP
Making sugars requires several cycles


Rubisco - the enzyme that dominates carbon fixation

- Most abundant enzyme in the world
- Most CO2 converted into biomass is fixed by Rubisco
- Large molecule : 16 polypeptides : 8 active sites
Rubisco: the clunky and slow carbon fixer
- Only 3-10 reactions per second
- 20% error rate
- gets worse at high temperatures
- Also reacts with O2
- O2 reaction called Photorespiration
- uses ATP and NADPH to make CO2

C3 photosynthesis how did we get here…
- Evolution doesn’t always create the best solution
- Photorespiration is completely wasteful
- 2x energy to produce the same amount of sugar than if Rubisco only reacted with CO2
- Rubisco evolved high affinity for CO2 (80x > O2)
- compare to the composition of the atmosphere…
- compare to global warming…

Rubisco substrate choice


Photorespiration inpacts food security

Rubisco and plant evolution: How did we get here?

Plant evolution tied to atmospheric shifts in CO2:O2


Why has Rubisco not been replaced?
- Evolution of enzymes difficult
- trade-offs between activity and stability
- Removing oxygen reaction breaks Rubisco
- selection against modifying mutations
- RubisCO → RubisC???
- requires a decrease in fitness
- plants are stuck with Rubisco
- Natural selection increased affinity for CO2

Evolution tinkers with other parts of photosynthesis

- Improved and recombined existing parts and pieces
- work around the photorespiration issue
- In response to environmental change
- lower [CO2] & higher [O2]
- high temperatures
- water limitations
- Evolution of new photosynthesis pathways
- Allowed plants to exploit new habitats
Plants and humanity: Why does photorespiration matter?
- Consumes 32% of ATP and 28% of NADH in C3 leaves
- Crop productivity improvement not sufficient to meet 2x food production by 2050
- Photorespiration decreases US soybean and wheat yields by 36% and 20%
- loss of 148 trillion potential calories
- would feed 203 million people for a year
